A decrease in external quantum efficiency (EQE) of commercial LEDs based on MQW InGaN/GaN at wavelengths of 445, 530 and MQW AlGaN/GaN at 280 nm was experimentally studied in the standard aging mode at direct current. The decrease in EQE (regardless of the radiation wavelength) is found out to occur due to cooperative phenomena developing in 1–2 quantum wells (QWs) located in the space charge region (SCR) around the p–n junction, as well as in most of the QWs outside of SCR. It is shown that the inhomogeneous flow of current in these regions leads not only to the transformation of defects localized at heteroboundaries in the SCR and in lateral inhomogeneities in the alloy composition outside of SCR as well as in the extended defects, but also to a change in the alloy composition.

The properties of graphene chips with low reproducibility (LR) after photolithography (PLG) and graphene functionalization have been studied. It is shown that the introduction of additional cleaning after PLG can significantly increase the reproducibility of the parameters of processed graphene in biosensors. The use of dilute PBS solutions for virus detection makes it possible to increase the relative concentration sensitivity of biosensors by several times.

The zone fluctuation potentials (ZFPs) in quantum wells located in the space charge region (SCR) of the p–n junction and the lateral ZFPs in quantum wells outside the SCR in blue, green, and UV LEDs based on nitrides have been experimentally determined. Green LEDs were used as an example to show that the low external quantum efficiency (EQE) of LEDs at the maximum correlated with an increase in the ZFP and disordering of heteroboundaries in quantum wells located in the SCR. The EQE at the maximum decreased because charge carriers were captured by charged centers localized at disordered heteroboundaries. The lateral ZFP in quantum wells located outside the SCR was the main parameter determining the decrease of the EQE from the moment the p–n junction opened until current densities reached 30–40 A/cm2.

A prominent source of charge carrier losses due to non-radiative recombination in AlGaN QWs, caused by the presence of charged centers localized at disordered hetero in-terfaces, has been experimentally revealed. It was found out that the spectral density of current low-frequency noise, which carries integral information about single defects and a defect sys-tem, is an order of magnitude higher in AlGaN QWs than in effective blue InGaN/GaN QWs. Thus, non-radiative recombination losses are still the source responsible for the low quantum efficiency of ultraviolet LEDs.

The paper describes the results of optimizing rapid thermal annealing (RTA) of ohmic contacts to AlGaN:Si layers with a high aluminum content (70 mol%) and various electron concentration. The contact characteristics were measured using the transmission line method (TLM). It has been found that for highly doped Al0.7Ga0.3N:Si layers (>1018cm-3), the RTA annealing of Ti(25nm)/Al(80nm)/Ti/Au contact at a temperature 900 & DEG;C for 60 s makes it possible to obtain the minimum contact resistance of 8 & omega;xmm and specific contact resistivity of 9x10-4 & OHM;& BULL;cm2 with high uniformity over the surface of a 2-inch substrate. For lightly doped Al0.7Ga0.3N:Si layers (<1017 cm-3), almost the same contact characteristics can be achieved at a higher RTA temperature of about 1000C and an increase in the thickness of the Al contact layer to 250 nm.

The contribution of several mechanisms into the external quantum efficiency (EQE) droop in green InGaN/GaN LEDs over a temperature increase from 300 to 400 K is clarified. One of them is the ionization of atoms localized at disordered hetero-interfaces in InGaN/GaN MQWs situated at the depletion region around a p-n junction at j < 10 A/cm2 and U < Utr (turn on voltage). The ionized atoms capture tunneling charge carriers, which leads to EQE decrease. Another mechanism is the capture of charge carriers tunneling in 3D spaces of MQWs situated outside of a depletion region at U > Utr and 10 A/cm2 < j < 30 A/cm2. Grow-ing thermalized carriers concentration reduces the band fluctuation potential which results in vertical diffusion transport of carriers and crowding effect.

The well-known effect of the local interaction between graphene and photoresist (LIGF) during the creation of biosensors is shown to lead to non-uniform distribution of compressive stresses, which deteriorates the adsorption properties of graphene, parameter reproducibility, and detecting ability of influenza B and SARS-Cov-2 biosensors. It is also shown that controlling the occurrence of LIGF areas on a graphene surface by atomic force microscopy or introducing a protective layer between graphene and photoresist can minimize the non-persistent effect of LIGF. The results of influenza B and SARS-CoV-2 imaging on the graphene surface in biosensor chips in a scanning electron microscope are presented.

The obtained experimental results allow us to clarify the nature of mechanisms related to the presence of cations in disordered InGaN alloy and hetero-interfaces. The capture of charge carriers by cations reduces the external quantum efficiency (EQE) in green MQWs at j < 10 A/cm2. The EQE droop phenomenon caused by smoothed out lateral potential fluctuations occurs at j > 10 A/cm2. At j > 40 A/cm2 the droop associated with interactions between charge carriers and dislocations and grain boundaries takes place.

In this work, the modification of the surface parameters of graphene chips after electrolysis treatment in a NaClO 4 aqueous solution has been studied. Two electrolysis modes have been analysed. In the first one, a negative potential (-0.2 V) is applied to the graphene chips, while in the second one the potential is positive (0.8 V). Investigation using a number of techniques including atomic force microscopy, Kelvin probe force microscopy, Raman spectroscopy, measurements of current-voltage characteristics and low-frequency noise has shown that the electrolysis mode with application of a positive potential on graphene chips decreases the 1/ f noise and allows one to obtain a uniform surface potential distribution while leaving the graphene structure undamaged. The results of this study help to understand the efficiency and reproducibility of the procedure for electrolysis treatment of graphene chips.

Temperature-dependent decline in the external quantum efficiency (EQE) of blue light-emitting diodes, which is at a maximum at j < 10 A/cm 2 and becomes stronger with temperature increasing to 400 K, is due to the buildup of the loss for the nonradiative recombination upon carrier tunneling that involves traps and phonons. As the p – n junction becomes open at j > 40 A/cm 2 the decline in the external quantum efficiency in the continuous-wave and pulsed modes is determined by the loss in nonequilibrium filling by delocalized carriers of states associated with the lateral inhomogeneities in the composition of the solid solution outside the space-charge region, as well as by the loss due to the interaction of delocalized carriers with extended defects.

The contribution of several mechanisms to the external quantum efficiency (EQE) droop in blue InGaN/GaN LEDs occur at different current densities j and voltages in MQWs situated inside and outside of a depletion region around p-n junction. It is clarified that an increase in EQE droop at 300-400 K (j < 10 A/cm2) is due to non-radiative losses related to an enhancement in trap-assisted tunneling. It is also associated with a growth in the concentration of delocalized carriers. The main source of the EQE droop under direct current and at pulse mode when j > 30 A/cm2 is non-equilibrium filling of lateral regions of different size within MQWs placed outside of depletion region by delocalized carriers activated by injection when voltage exceed a threshold value (U > Uth). This leads to a decrease in localized potential and to the blue shift of EQE maximum over wavelengths followed by the EQE droop.

The thermal droop of external quantum efficiency (EQE) at maximum in blue InGaN/GaN LEDs at j < 10 A/cm2 is caused by increasing losses related to non-radiative recombination due to carrier tunneling with the assistance of phonons and traps, enhancing by a temperature growth up to 400 K. When a p-n junction opens at j > 40 A/cm2, the EQE droop under direct current and at pulse mode is due to the losses associated with non-equilibrium filling of the states related to lateral alloy non-uniformities in quantum wells situated outside of the depletion region by delocalized carriers as well as the losses due to the interactions between delocalized carriers and extended defects.

Graphene is considered as a promising candidate for manufacturing of sensors due to its extreme sensitivity to molecule absorption. In this work, we show the connection between the electrical and optical properties of epitaxial graphene chips grown on 4 H -SiC and intended for the production of protein-based sensors. Using of a complex of techniques, including Raman spectroscopy, atomic force microscopy, Kelvin probe microscopy, study of I-V characteristics and low-frequency noise, it is shown that the character of frequency dependence of the spectral density of voltage fluctuations and its value at a frequency of 1 Hz can be used for classification and selection of graphene chips for their application as sensors. Classification of the graphene chips will allow more efficient development of graphene-based biosensors.

Some green light emitting diodes (LEDs) based on GaN/InGaN multiquantum-well (MQW) structures exhibit strong frequency and temperature dependence of capacitance and prominent changes in capacitance–voltage profiles with temperature that make it difficult to obtain reliable deep level transient spectroscopy (DLTS) measurements. DLTS performed at low probing signal frequency and with constant capacitance between the measurements by controlling applied bias mitigates these issues. This allows measurement of deep electron and hole traps in specific quantum wells (QWs) in the MQW structure. The dominant electron and hole traps detected have levels near Ec− (0.45–0.5) eV and Ev+ (0.6–0.63) eV. Their density increases significantly after aging for a long period (2120 h) at high driving current and elevated temperature. The reason for the observed anomalies in DLTS spectra of these green LEDs is the high density of states in the QWs with activation energies near 0.08, 0.12–0.14, and 0.3 eV, detected in admittance spectra, and, for the 0.08 eV and 0.3 eV, these states are likely related to defects.

AbstractIt is established experimentally that noticeable changes in the I – V characteristics and low-frequency noise in 4 H -SiC pin diodes irradiated by electrons with an energy of 0.9 MeV are observed after doses of Φ ≥ 1.4 × 10^15 cm^–2. The currents in the forward and reverse branches of the I – V characteristics vary nonmonotonically at voltages lower than 2 V with increasing dose, which is explained by the interaction between the excited electronic subsystem and metastable defects. In this case, a steady increase in the ideality factor and the series resistance of diodes in the region of exponential growth of the I – V characteristics at voltages exceeding 2 V is observed. The reliable operation of microwave devices with low-noise 4 H -SiC pin diodes under conditions of electron irradiation is possible up to a cumulative dose of Φ ≤ 10^15 cm^–2. In microwave devices, the level of low-frequency noise in which is irrelevant but the stabile regime of parameters is of importance, the dose can be increased to Φ ≈ 8 × 10^15 cm^–2.

It is established experimentally that noticeable changes in the I–V characteristics and low-frequency noise in 4H-SiC pin diodes irradiated by electrons with an energy of 0.9 MeV are observed after doses of Φ ≥ 1.4 × 1015 cm–2. The currents in the forward and reverse branches of the I–V characteristics vary nonmonotonically at voltages lower than 2 V with increasing dose, which is explained by the interaction between the excited electronic subsystem and metastable defects. In this case, a steady increase in the ideality factor and the series resistance of diodes in the region of exponential growth of the I–V characteristics at voltages exceeding 2 V is observed. The reliable operation of microwave devices with low-noise 4H-SiC pin diodes under conditions of electron irradiation is possible up to a cumulative dose of Φ ≤ 1015 cm–2. In microwave devices, the level of low-frequency noise in which is irrelevant but the stabile regime of parameters is of importance, the dose can be increased to Φ ≈ 8 × 1015 cm–2.

AFM study of surface morphology in green LED structures and multifractal analysis allowed us to reveal a relationship between step-meandering morphology quantitatively characterized by a multifractal parameter, the degree of disorder, and features of In incorporation in InGaN/GaN MQW structures. These features manifest themselves in a shape of the distribution of peak external quantum efficiency values with wavelengths (DPEW) in the current range of 0.1–1000 mA. High values of degree of disorder result in spinodal alloy decomposition and/or local In enriched regions in InGaN alloy and in a twofold decrease in EQE values in green LEDs.

It has been clarified that the distribution of peak EQE values over wavelengths in the range of currents 0.1 − 1000 mA in InGaN/GaN MQW contains information on the In distribution. The correlation has been found between the EQE values of green polar InGaN/GaN LEDs and the In distribution and redistribution in InGaN/GaN MQW under the influence of growth conditions, accelerated aging, and electron irradiation.

AbstractA fractal-percolation system that includes extended defects and random fluctuations in the alloy composition is formed during the growth of device structures based on Group-III nitrides. It is established that the specific features of this system are determined not only by the growth conditions. It is shown that the diversity of the electrical and optical properties of InGaN/GaN LEDs (light-emitting diodes) emitting at wavelengths of 450–460 and 519–530 nm, as well as that of the electrical properties of AlGaN/GaN HEMT (high-electron-mobility transistor) structures, is due to modification of the properties of the fractal-percolation system both during the growth process and under the action of the injection current and irradiation. The influence exerted by these specific features on the service life of light-emitting devices and on the reliability of AlGaN/GaN HEMT structures is discussed.

Secondary-ion mass spectrometry and Rutherford proton backscattering have been used to measure the concentration profiles of nitrogen atoms and examine the defect structure of epitaxial GaAs layers implanted with 250-keV N+ ions at doses of 5 × 1014–5 × 1016 cm–2. It was found that no amorphization of the layers being implanted occurs at doses exceeding the calculated amorphization threshold, a concentration of point defects that is formed is substantially lower than the calculated value, and a characteristic specific feature of the defect concentration profiles is the high defect concentration in the surface layer.